1. Field of the Invention
The present invention relates to aluminum fine powder suitable for use in an aqueous phosphate-based coating composition which takes advantage of a sacrificial corrosion inhibiting effect of aluminum and to an aqueous chromium-free corrosion inhibiting coating composition which does not contain harmful chromium components and which is formed with the aluminum fine powder suspended in an aqueous phosphate-based bonding solution, in order to inhibit corrosion of a metal surface, especially a surface of iron-based material.
2. Description of the Related Art
There are a variety of conventional corrosion inhibiting coating compositions depending on mechanisms of corrosion inhibition and based on corrosion inhibiting components. As such corrosion inhibiting coating compositions, coating compositions that are excellent in a corrosion inhibiting effect, taking advantage of a sacrificial corrosion inhibiting effect of zinc or aluminum, have been known as in the case of plating. Further, there are different kinds of bonding solutions (binders) for the corrosion inhibiting coating compositions, which are roughly classified into two categories, those with an inorganic aqueous bonding solution and those with an organic bonding solution. Well-known corrosion inhibiting coating compositions using the inorganic aqueous bonding solution include an alkali-silicate type of corrosion inhibiting coating composition such as a zinc-rich paint and a phosphate-based corrosion inhibiting coating composition such as a Sermetel coating composition available from SERMATECH INTERNATIONAL, INC. of the U.S. taking advantage of the sacrificial corrosion inhibiting effect of aluminum which is used for surface treatment of turbine blades of aircraft engines.
Many of these corrosion inhibiting coating compositions usually contain hexavalent chromium components that have a specific corrosion inhibiting effect. For example, a fundamental technology of the Sermetel coating composition from SERMATECH, INC. is disclosed in U.S. Pat. No. 3,248,251 filed in 1963 by Teleflex Incorporated that is a parent company of the SERMATECH. Although the patent describes that molybdate has the same corrosion inhibiting effect as that of chromate, hexavalent chromium (chromate ion) having a significant corrosion inhibiting effect is actually included in the Sermetel coating composition as an essential component.
Hexavalent chromium in the corrosion inhibiting coating composition would lead to formation of a conversion coating on a metal surface or to passivation of the metal surface. That is, the hexavalent chromium component in the Sermetel coating composition forms the conversion coating even on a particle surface of aluminum fine powder which is suspended in the coating composition so as to effectively prevent a reaction generating a hydrogen gas between an acidic phosphate-based bonding solution, the pH of which is around 3, and the aluminum fine powder. Therefore, it becomes possible to achieve a several-months pot life of the corrosion inhibiting coating composition.
Further, the corrosion inhibiting effect of the hexavalent chromium component also effects a metal substrate to be coated as well. The conversion coating or passivation coating including hexavalent chromium is formed on a surface of the metal substrate and prevents a reaction between the acidic phosphate-based bonding solution and substrates (hydrogen is generated when steel products are dipped). This conversion coating has the effect of protecting the substrate material from rust even after the coating has been baked. The Sermetel coating composition has excellent properties resulted from combining the sacrificial corrosion inhibiting effect of aluminum and the corrosion inhibiting function of the hexavalent chromium component and also has a reputation as an excellent corrosion inhibiting coating with a heat resistance up to around 600xc2x0 C.
However, it has been commonly known that the hexavalent chromium component is carcinogenic, and even if trivalent chromium is used, it is not easy to prevent oxidation of trivalent chromium to hexavalent chromium. Therefore, it is preferable to use a corrosion inhibiting coating composition that is completely free of chromium components with consideration of their influences on the global environment. The first-ever non-chromium corrosion inhibiting coating composition, which is completely free of chromium components, has been developed by Solar Turbines Incorporated of the U.S. and is disclosed in U.S. Pat. No. 5,242,488. Thereafter, U.S. Pat. No. 5,478,413 titled xe2x80x9cEnvironmentally friendly coating compositionsxe2x80x9d was filed from the SERMATECH INTERNATIONAL INC., which is an manufacturer of the Sermetel coating, and issued.
A problem in practical utilization of these non-chromium corrosion inhibiting coating compositions is that aluminum fine powder dispersed in the coating composition reacts with an acidic phosphate-based bonding solution to produce hydrogen, so that the coating composition becomes unusable within 1-2 days due to an increase in its viscosity and has a short pot life. The inventors have assumed that the pot life can be increased by forming an overcoat, alternative to a conversion coating formed by hexavalent chromium, on a surface of the aluminum fine powder, and now consequently found a solution as a result of various studies.
An object of the present invention is to provide aluminum fine powder having an overcoat, alternative to a conversion coating formed by harmful hexavalent chromium, on its surface and a phosphate-based aqueous chromium-free corrosion inhibiting coating composition having a practical pot life in which the aluminum fine powder is suspended in a bonding solution.
According to the present invention, the surface of the aluminum fine powder is treated with a hydrophilic coupling agent. A titanium-coupling agent can be used as the hydrophilic coupling agent.
Further, in the present invention, the aluminum fine powder which is surface treated with the hydrophilic coupling agent can be obtained by preparing ethyl alcohol to which 3 to 12% by weight of water is added, dissolving 0.3 to 1.5 parts by weight of the hydrophilic coupling agent relative to 100 parts by weight of the aluminum fine powder to be added in this solution, suspending the aluminum fine powder to be treated in this solution, forming a thin film of the coupling agent on a particle surface of the aluminum fine powder by heating and keeping the solution at 50 to 70xc2x0 C. for 2 to 7 hours with the aluminum fine powder suspended therein, filtrating the solution which contains hydrolytic products of the coupling agent to recover the aluminum fine powder, rinsing the recovered aluminum fine powder with ethyl alcohol having a purity of 99% by weight or more, and heating the rinsed aluminum fine powder to a temperature which is lower than a boiling point 78.3xc2x0 C. of ethyl alcohol to dry it under reduced pressure.
An aqueous corrosion inhibiting coating composition according to the present invention is a composition in which 25 to 40% by weight of the aluminum fine powder which is surface treated with the hydrophilic coupling agent is suspended in the acidic phosphate-based aqueous bonding solution. In this case, a titanium-coupling agent can be used as the hydrophilic coupling agent.
In the above-described case, the phosphate-based aqueous bonding solution can contain phosphate ions, aluminum ions, zinc ions, and two or more kinds of alkaline earth metal ions, and further contain 0.2 to 1% by weight of polyphosphate ions.
A surface of the aluminum fine powder used in the present invention is treated with the coupling agent and a surface of the particle is covered with a thin overcoat of the coupling agent. Although there are many kinds of coupling agents, it has been found that the only limited kinds of coupling agents can be effective for this purpose. An effective coupling agent is within a class in which the treated surface exhibits hydrophilic property, so that making the particle surface of the aluminum fine powder hydrophilic and facilitates its dispersion into the aqueous phosphate bonding solution. When other coupling agents are used, the treated surface exhibits hydrophobic property, so that it is essential to take advantage of an aid of a dispersion agent or surfactant in order to suspend the aluminum fine powder into the aqueous bonding solution. Therefore, it is difficult to sufficiently disperse the hydrophobic powder into the aqueous bonding solution.
Treatment of the aluminum fine powder with the coupling agent results in formation of an overcoat of the coupling agent on a surface of the aluminum fine powder, and reduces surface activity of the aluminum fine powder for a reaction. Specifically, a reaction in which hydrogen generates when the aluminum fine powder is dispersed into the acidic phosphate chromium-free bonding solution can be prevented, and the pot life of the corrosion inhibiting coating composition being stored in a refrigerator can be increased from several days to nearly one months, and safety of storing the aluminum fine powder which is flammable can be improved.
Considering influence on the global environment, the corrosion inhibiting coating composition is preferable to be free of chromium components, to be an aqueous composition, and to barely release volatile organic components (VOCs) such as organic solvents.
There are two types of commercially available aluminum fine powder, that is, an air-atomized powder being atomized by compressed air and a gas-atomized powder being atomized by non-oxidative compressed gas. In the former, a surface of the powder particle is covered with a relatively thick oxide film and constituted by oval particles being deformed by influence of the oxide film, and in the latter, a surface of the powder particle is covered with a thin oxide film and constituted by sphere particles. Since a particle size distribution of the atomized aluminum powder is usually considerably wide, classified powders depending on their uses are commercially available, and for example, there is aluminum fine powder having a mean particle diameter of as small as about 1 xcexcm. Also, aluminum fine powder constituted by scale-like particles made by processing the atomized powders is commercially available, and the typical scale-like particle has a width of the order of 10 xcexcm and has a thickness of the order of 0.1 xcexcm.
An aluminum dihydrogen phosphate solution is preferable to be used as a main raw material of the phosphate-based aqueous bonding solution for an aqueous corrosion inhibiting coating composition according to the present invention. The aluminum dihydrogen phosphate solution having a concentration of about 20% by weight is used as a binder for refractories or the like and is commercially available, so that it is preferable to use the solution in order to save time and effort to dissolve powder of the phosphate into water.
When a dispersion agent is added to the binder solution during preparation of the aqueous corrosion inhibiting coating composition of the present invention, the aluminum fine powders are well dispersed into the bonding solution and enables to form a homogeneous coated film, but generate foams which cause defects in the coated film hardly disappears. Therefore, it is preferable to decrease the foam by the combined use of an antifoaming agent and the dispersion agent.
Hydroxyethyl cellulose or hydroxypropyl cellulose is used for increasing the viscosity of an aqueous coating composition, and the coating composition can be viscous by adding the above-described cellulose compound in the form of a 4% by weight of aqueous solution. This kind of cellulose compound can be preferably used by previously preparing its aqueous solution having a concentration about 4% by weight and by mixing the solution with the bonding solution.
Glycerin has an effect of increasing the viscosity, and a function of regulating a drying rate of the coating composition (decreasing the drying rate). Previously making a mixed solution of glycerin with a dispersion agent or antifoaming agent which does not easily dissolve in water facilitates mixing these with an aqueous bonding solution.
The corrosion inhibiting coating composition is usually used as an undercoat, and it is common to apply a top coat on the corrosion inhibiting coated film in order to further improve the corrosion inhibiting performance of the coated film.
As a corrosion inhibiting coating composition for screws or fasteners, there is a usage for precision screws, which requires a thin film of around 8 xcexcm in thickness. To form such a thin-coated film, it is preferable to use aluminum fine powder having a mean particle diameter of around 1 xcexcm or scale-like aluminum fine powder.
A sacrificial corrosion inhibiting effect is such an effect when both an easily ionizable metal belonging to the former part (less noble) of the ionization series is in electrical connection with a hardly ionizable metal belonging to the latter part (noble) of the ionizarion series are present in an electrolyte solution such as salt water, the easily ionizable metal is preferentially ionized (oxidized), so that the other metal is protected from oxidation.
Burnishing treatment performed on the corrosion inhibiting coated film including aluminum fine powder as filler is a treatment for providing electrical connection (electrical conductivity) between particles of the aluminum fine powder in the coated film and between the aluminum fine powder particles in the coated film and a steel substrate. That is, this treatment is a kind of blast treatment in which abrasive grain particles of 320 meshes are moderately blasted onto a surface of the coated film. This burnishing treatment ensures to keep the sacrificial corrosion inhibiting effect of the coated film.
A purpose of adding water into an ethyl alcohol solvent which dissolves a coupling agent is to hydrolyze the coupling agent and deposit the coupling agent on a surface to be treated so as to form a thin film of the coupling agent. A typical amount of water to be added to the ethyl alcohol, which is used for the coupling treatment, is usually around 10% by weight, but this amount can be changed as appropriate. Treated aluminum fine particles adhering on a surface of a used vessel can be washed off by ethyl alcohol as appropriate and recovered within a batch. Although a coupling treatment conducted for a sufficiently long time can form a coupling film that completely covers the surface of the treated powder particles, this treatment is preferable to be done within 7 hours, which allows the treatment to be completed within one day.
A temperature of coupling treatment is preferable to be 50 to 70xc2x0 C., which is lower than the boiling point of ethyl alcohol, 78.3xc2x0 C., so as to minimize evaporation of ethyl alcohol. A rotary evaporator was used for coupling treatment in the examples described hereinafter, but this coupling treatment may also be conducted in a tank fitted with a stirrer, which temperature is thermally controllable. The aluminum fine powder treated with the coupling agent may be rinsed by ethyl alcohol of 99% by weight or more to remove an excess part of colloidal substances produced by hydrolysis of the coupling agent.
The aluminum fine powder, which has been rinsed by ethyl alcohol after the coupling treatment, can be quickly dried. The wet cake of aluminum fine powder is heated in a dryer, then ethyl alcohol is evaporated in an evacuated vessel fitted with a water jet pump, and the powder can be dried without discharging vapors from ethyl alcohol in a room.
Since mixing of organic solvents such as acetone or ethyl alcohol with the phosphate-based bonding solution may cause degeneration of the bonding solution and may change its viscosity (for example, increases viscosity), the aluminum fine powder wetted with ethyl alcohol used in the coupling treatment is preferable to be dried sufficiently. Sufficient drying of the powder in a dryer kept at around 70xc2x0 C. for more than several hours can increase adhesion of the coupling film to the surface of the fine powder particle. After drying, the aluminum fine powder can be recovered as a cake that can easily be made into a powder.
An amount of aluminum fine powder to be suspended in the phosphate-based aqueous solution is determined with consideration of a concentration of the aqueous phosphate-based bonding solution to be used (with consideration of a volume ratio of a dried bonding solution and aluminum fine powder in the corrosion inhibiting coating film), and is preferably 25 to 40% by weight of the corrosion inhibiting coating composition.
The phosphate-based bonding solution becomes a water-insoluble solid (vitreous) after baking at 300xc2x0 C. or more. When double-coating is performed, a temperature for baking the first coated film should preferably be around 250xc2x0 C., but a temperature for baking the second coated film must be a given temperature to obtain a water-insoluble coated film. A high baking temperature can reduce the time required for the baking treatment. However, when the baking temperature rises to a temperature (around 600xc2x0 C.) at which the aluminum fine powder may be oxidized, a thermit reaction may occur in which the adjacent oxide substances are reduced and the aluminum fine powder is oxidized.
The aqueous phosphate-based bonding solution contains a lot of phosphate ions and aluminum ions due to the use of an aluminum dihydrogen phosphate solution as a main raw material. In order to improve the corrosion inhibiting performance and to reduce reaction activities between the bonding solution and the aluminum fine powder, various kinds of metal ions are introduced to the bonding solution. For example, it is assumed that the presence of zinc ions leads to formation of a conversion coating and the corrosion inhibiting performance of the coating composition will improve. On the other hand, it is assumed that the presence of alkaline earth metal ions such as magnesium ions weakens the acidity of the bonding solution to suppress the reaction of bonding solution with the aluminum fine powder.
In the case of the bonding solution as described in the examples below, the aluminum fine powder was added to the starting solution and heated in advance to allow the reaction which generates hydrogen to proceed as much as possible, and the concentration of aluminum ions in the bonding solution is increased to a level of saturation, in order to minimize the reactivity of the bonding solution with the aluminum fine powder.
In this reaction, metal ions such as magnesium ions, which have once dissolved in the solution, is precipitated out. Properties and amounts of the precipitate vary depending on the kind and amount of alkaline earth metal ions included in the solution, and when a lot of magnesium ions are included in the solution, a large amounts of viscous precipitate will appear. Introduction of calcium ions or strontium ions can decrease the amount of the precipitate. However, these metal ions can not dissolve in large amounts because of the limitations of their solubility and the time required for their dissolution. Therefore, these metal ions are preferably added to the bonding solution in the form of carbonates or hydroxycarbonates, which are easy to be dissolved.
Addition of a small amount of sodium hexametaphosphate (a kind of polyphosphates), which is assumed to have an effect as an inhibitor for suppressing oxidation of the metal, to the bonding solution (0.2 to 1% by weight) can decrease the amount of the precipitate and increase the viscosity of the bonding solution. Further, production of the precipitate, which may easily occur in the stored bonding solution, can be avoided.
Various well-known inhibitors are additives for suppressing oxidation reaction of the metal, and an introduction of some inhibitors possibly improve the corrosion inhibiting characteristics of the corrosion inhibiting coating composition. However, attention should be paid when using other inhibitors which are not compatible with the aqueous phosphate-based bonding solution, because the inhibitors may not dissolve in the solution or the amount of precipitate may be increased.